Linux and RTAI for Building Automation

This easy-to-deploy Web-based control system uses standard phone wiring and can manage any device that supports an infrared remote control.

Real-Time Tasks

The main control task, the network access-control task and the software
driver for the physical layer of the RS-485 network are the tasks that
run in the real-time executive.
An RS-485 driver was developed for RTAI. This driver is similar to
any other serial driver, except for the 9th-bit protocol used in
this application, as described above.

The other real-time task is the network access-control task, which is
in charge of periodically sending packets to each network node. This
packet can be a command to generate an IR signal, a poll to see if the
node is active or a command to the microcontroller to transmit the actual
room temperature. The node answers with an acknowledgement to the first
two types of packets and with the actual room temperature to the last
one. The information about the actual state of every node is available
to the main control task, which in turn informs the user interface if
a node fails.

The main control task, using information retrieved from
the database, operates the air-conditioning equipment in
the building, as programmed. This task also can receive
instructions from the user interface that overrides the
programmed configuration, using two RT-FIFOs. RT-FIFOs are an
interprocess communication routine for communication between
real-time tasks and normal Linux tasks. To communicate with the
PostgreSQL database, a Linux dæmon was developed.
This dæmon communicates with
the main control task using two more RT-FIFOs. An additional
important function of this dæmon is to send to the main control
task the system date and time; no support for
reading it exists in RTAI.

The developed system sends commands to the air conditioners,
eliminating the need for local remote controllers. We do not interfere with
the air-conditioner temperature control system, nor do we touch any
internal circuitry. Each air
conditioner has its own temperature control system built-in,
and the temperature sensor in each microcontroller supervises that the
equipment is working fine. Figure 4 shows the microcontroller board installed.

Figure 4. The complete microcontroller board as installed.

Linux Tasks

The Linux tasks are in charge of presenting the user interface through
a Web server and running the PostgreSQL database engine, which is the
main data repository. As described above, another Linux side task is a
dæmon used for the RTAI main control task to access the system date/time
and the database.

The user interface is simple. The first page presents information
about the actual state of each air conditioner. Every type of user
can access this page. In order to change the program or send commands
to a particular air conditioner, the system asks for a user name and
password. PHP is used to generate the Web pages dynamically to present
the information retrieved from the database.

In the PostgreSQL database, the system stores general information about the air
conditioners, such as BTU, location, brand and microcontroller network node
address; the programmed operations; and the IR commands
needed to operate each air conditioner.

IRC Command Interface

An important part of the system is the module that reads the air-conditioner remote controller signals and stores the information,
associated with the corresponding equipment, in the database to
reproduce it using the networked microcontrollers. This module is
used
only when adding a type of air conditioner
that has a different brand and/or different remote controller commands.

Two tasks are part of this module: the first is a real-time
task that reads the IR signal. The LIRC Project as well as
the Ripoll and Acosta paper in the on-line Resources, present detailed
information about IR remote controllers and sample
implementations using normal Linux and RTLinux, another
real-time executive for Linux. The other task for this
module is the user interface that runs on Linux. The two
tasks communicate using an RT-FIFO.

Due to the small amount of RAM available in the microcontroller and
the long IR signal duration, an important function of this software is
to help the user obtain repetitive patterns within the different
IR remote controller signals associated with each button or combination of
buttons. These patterns are coded in the firmware of the microcontroller and
are used to reconstruct the command to control the equipment. For example, if
there are ten different patterns, the information sent to the appropriate
microcontroller in the network is something like: repeat pattern one ten
times, then pattern two three times and so on, until the complete command
is reconstructed.
This technique has the advantage of using fewer resources for signal
reconstruction. The disadvantage is
the software of the microcontroller needs to be changed to
introduce the patterns of the newly added equipment whenever a new air
conditioner is introduced.